PH‐Responsive Supported and Unsupported Gold Nanocrystals

Abstract

AbstractThe pH‐responsive zwitterionic amphiphile (C18Tyr) was used for the functionalization of unsupported Au nanocrystals, a SiO2 support, and SiO2‐supported Au nanocrystals in a biphasic water/chloroform system, for useful supported nanomaterials. The solubility of C18Tyr was controlled by adjusting the pH of the water/chloroform system; at pH 8.0 and 3.5, almost all of the C18Tyr dissolved in the water and chloroform phases, respectively. This pH‐dependent solubility control is due to protonation/deprotonation of the amino and carboxyl groups of C18Tyr. The unsupported Au nanocrystals in C18Tyr/water/chloroform were reversibly transferred between the water and chloroform phases by pH control, but the phase transfer yield was low due to the aggregation and deposition of Au nanocrystals. Therefore, we prepared pH‐responsive supported nanocrystals, as they are known to have high stabilities compared to unsupported nanocrystals. The SiO2 support, without Au nanocrystals, was reversibly dispersed and precipitated in the C18Tyr/water/chloroform system by pH control, and not reversibly transferred between the water and chloroform phases. SiO2‐supported Au nanocrystals were also reversibly dispersed and precipitated as was observed for the SiO2 support. Therefore, the dispersion‐precipitation response of the SiO2‐supported Au nanocrystals is due to the SiO2 support, not the Au nanocrystals. Furthermore, no loss of Au nanocrystals was observed due to the improved stability of the Au nanocrystals supported by this method. These results are very useful for preparation and application of externally responsive supported nanocrystals with high stabilities.